1. Field of the Invention
The present invention relates to a method of manufacturing toner carrier, a device for manufacturing toner carrier, toner carrier, a development agent, a process cartridge, a method of forming images, and an image forming apparatus.
2. Description of the Background Art
In electrophotography, images are formed by forming a latent electrostatic image by electrostatic charge on an image bearing member made of photoconductive materials, etc., attaching charged toner particles to the latent electrostatic image to obtain a visual toner image, transferring the toner image to a recording medium (typically paper), and fixing the toner image in place on the recording medium.
In recent years, the technologies regarding photocopiers and printers employing electrophotography have been rapidly developing from monochrome to full color, thereby expanding the full color market.
In color image formation employing full color electrophotography, three color toners containing toners of three primary colors of yellow, magenta, and cyan or four color toners including color of black in addition to the three color toners are laminated to represent all the colors.
Therefore, to obtain vivid full color images with excellent color representation, the surface of the fixed toner image is smoothed in some degree to reduce light scattering.
As a result, most of the image gloss of images produced by a typical full color photocopier, etc., ranges from moderate gloss to high gloss, e.g., 10% to 50%.
In general, as a method of fixing dry toner images on a recording medium, a contact heating fixing method is commonly used in which a heated roller or belt having a smooth surface is pressed against the toner.
This method is advantageous in terms of heat efficiency, fixing speed, and ability to import gloss and transparency to color toner.
However, since melted toner is peeled off from the surface of a heated fixing member after the melted toner contacts the surface under pressure, part of the toner image may remain attached to that surface, resulting in transfer of the attached toner to another recording medium on which another toner image is formed. This phenomenon is referred to as hot offset.
To prevent hot offset, a heating fixing member having a surface formed of materials having excellent releasability such as silicone rubber and fluorine resins is typically used and in addition a releasing oil such as silicone oil is applied to the surface of the heating fixing member.
This method is extremely effective to prevent hot offset of toner but requires a device to supply the releasing oil, thereby not shrinking but expanding the size of a fixing device. In an attempt to handle this drawback, with regard to monochrome toner, in addition to inclusion of a releasing agent such as wax in the toner, the viscous elasticity of the toner in melted state is improved by adjusting the molecular weight distribution of the binder resin to avoid fracturing of the melted toner from inside, which makes it possible to apply little or no releasing oil to the fixing roller.
Similarly, such an oil-free application method has become common in color toners to simplify the machines and make them more compact.
However, as described above, since the surface of a fixed image is required to be smooth to improve the color representation of color toner, toner in a melted state having a low viscous elasticity is suitable.
Consequently, color toner causes hot offset more easily than monochrome toner, which does not require gloss.
That means that it is more difficult to employ a method of applying little or no oil to the fixing roller in the case of color toner.
In addition, when toner particles containing a releasing agent are used, the attachability of the toner particles increases, thereby degrading the transferability of the toner to a transfer medium and resulting in contamination of triboelectric members such as toner carrier by the releasing agent in the toner particles, which leads to degradation of the chargeability and durability of the toner.
In addition, with regard to the toner carrier, to meet the increasing demand for more beautiful images, the size of the toner carrier particles is reduced while still being required to maintain the ability to continue to charge the toner.
On the other hand, as processing speeds increase, the stress on the toner carrier drastically increases.
Therefore, to improve the durability and the charging stability of the toner carrier, resin-coated toner carriers in which the surface of the core is covered with various resins are generally used.
However, development agents are usually subjected to stress caused by collisions between toner carrier particles due to stirring, friction between the development box and toner carrier particles, etc., which leads to the cover of the toner carrier particles being scraped or peeled off from the surface of the toner carrier particle core.
For example, when the cover of the toner carrier particle is scraped off and the inner cover exposed, the surface characteristics of the toner carrier particle tend to vary among the toner carriers particles, resulting in variance in the size of charge, which easily leads to toner fogging.
In addition, if the cover is deeply scraped or peeled off and the core exposes, the resistance of the toner carrier changes, which causes toner carrier attachment.
As one of the causes of the scraping and peeling-off of the cover of the toner carrier as described above, there is the solvent contained in the liquid cover that remains in the cover of the toner carrier.
That is, due to the solvent remaining in the cover of the toner carrier, the strength of the cover deteriorates so that the cover scrapes off easily.
Furthermore, solvent remaining between the core and the cover weakens the attachment between the core and the cover, which leads to peeling-off of the cover
In a typical external heating system using hot air or a heater, the external portion of the cover starts drying first while the solvent on the core side of the cover cannot ooze through the cover and tends to remain therein.
In an attempt to solve this problem, Japanese Patent No. 3133854 (JP-H05-341579-A) describes baking toner carrier with microwave radiation, after a liquid cover is applied to the core of the toner carrier, to improve the attachment between the core and the cover by evenly transferring heat from the inside of the cover.
However, in the JP-3133854-B1 (JP-H05-341579-A) mentioned above, microwave radiation is used only in the baking process, not in the coating process to remove the solvent in the cover. Due to solidification of the cover during the application process, the solvent tends to be enclosed on the side closer to the core.
Consequently, it is found that the remaining solvent is not sufficiently removed even using the microwave heating.